1. Field of the Invention
The present invention generally relates to a braking hydraulic pressure control apparatus for a vehicle and, more particularly, to an apparatus which is capable of anti-lock brake control and traction control over driving-wheel brakes.
2. Background Description
A braking hydraulic pressure control apparatus of the sort mentioned above is discussed in Japanese Patent Publication No. Hei. 5-65388.
In such a braking hydraulic pressure control apparatus, a switching valve means is actuated during normal braking operation. During normal braking operations and in the anti-lock brake control mode, a hydraulic passage connected to the discharge port of a pump is connected to a master cylinder, and an open passage connected to an inlet port of the pump is shut off from the master cylinder. When driving-wheel traction control is carried out, the switching valve means is actuated so that the open passage is connected to the master cylinder and the hydraulic passage is shut off from the master cylinder.
Prior to filling the master cylinder with brake fluid, a vacuum is drawn from the master cylinder side in order to remove air from the system. A three-port, two-position electromagnetic switching valve capable of switching between the following two positions has been employed as a switching valve means in the prior art. In the first position, the master cylinder is connected to the hydraulic passage, but is shut off from the open passage. In the second position, the master cylinder is connected to the open passage, but is shut off from the hydraulic passage. Consequently, in order to draw a vacuum, the three-port, two-position electromagnetic switching valve needs to be switched in two stages. In other words, a first vacuum is drawn when the three-port, two-position electromagnetic switching valve is positioned so that the master cylinder is connected to the hydraulic passage, but shut off from the open passage. Air can then be removed from the hydraulic pressure system between the driving- wheel brake and the master cylinder, and between the discharge port of the pump and the master cylinder. The three-port, two-position electromagnetic switching valve is then switched so that the master cylinder is connected to the open passage, but shut off from the hydraulic passage. A second vacuum is drawn from the hydraulic pressure system among the inlet port of the pump, the reservoir and the master cylinder. Although air can be sufficiently removed from the described braking control apparatus, the equipment utilized to remove the air tends to be complex because the valve position of the three-port, two-position electromagnetic switching valve has to be moved when removing air.